1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor substrate widely used as the substrate for a power device, a complex device or a VLSI. More particularly, it relates to a method of manufacturing a semiconductor substrate by directly bonding two silicon crystalline wafers to each other.
2. Description of the Related Arts
In a known method of manufacturing a semiconductor substrate by directly bonding two silicon wafers composed of silicon crystalline bodies, together first, each silicon wafer 11 composed of a silicon crystalline body is washed in a washing process shown in FIG. 6A. In this washing process, an oxide film 12 is formed on the surface of the silicon wafer 11.
The silicon wafer 11 thus formed with the oxide film 12 is then soaked in a diluted hydrofluoric acid (1 to 10 wt %) as shown in FIG. 6B for removing the oxide film 12 formed on the surface thereof. After that, it is subjected to oxidation treatment in a H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 solution. For example, it is soaked in a solution of [H.sub.2 SO.sub.4 (4):H.sub.2 O.sub.2 (1)] at 90.degree. C. for 3 to 5 min. Thus a thin oxide film 13 having a thickness of, for example, 50 .ANG. is formed on the surface of the silicon wafer 11 as shown in FIG. 6C. As a result, the surface of the silicon wafer 11 is made hydrophilic.
As shown in FIG. 6D, the silicon wafer 11 is then soaked in ultra pure water for about 30 min. Consequently, the surface of the silicon wafer 11 absorbs water molecules and is thus covered with OH groups. After that, as shown in FIG. 6E, two silicon wafers 111 and 112 thus treated are contacted with each other. Subsequently, as shown in FIG. 6F, they are heat treated to be thus bonded to each other.
In the above conventional method, the surface of the silicon wafer 11 is made hydrophilic and thus covered with OH groups as a result of formation of the thin oxide film 13 thereon by the oxidation treatment in a H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 solution. Thus, by contacting the surfaces of two silicon wafers 111 and 112 covered with the OH groups with each other, it is possible to bond them to each other with the aid of hydrogen bonds.
In the above, if the oxidation treatment in a H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 solution is not carried out, the surface of the silicon wafer 11 treated with the diluted hydrofluoric acid is not made hydrophilic, which makes it impossible to bond the two silicon wafers 111 and 112 to each other. Accordingly, the oxidation treatment in a H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 solution to form a thin oxide film 13 on the surface of silicon wafer 11 is indispensable.
However, the above oxide film 13 formed by the oxidation treatment in a H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 solution has the following disadvantages: namely, even if being heat treated thereafter, the oxide film is localized at the bonding interface and the vicinity thereof, thereby increasing the electric resistance in such a direction as passing through the bonding interface; and the localized oxide film brings the occurrence of crystal defects, which causes inconvenience in the characteristics of the device using this semiconductor substrate.